1. Field of the Invention
The invention relates to a process for the thermomechanical treatment of steel for torsionally-strained spring elements, the initial material being heated with a heating rate of at least 50 K/s and austenitized, and the product then being formed in at least one forming step with the formed product being quenched to below the martensite temperature to martensite and then tempered.
2. Description of Related Art
A process of the initially mentioned type is already known from German Patent No. DE 43 40 568 C2. In this known process, wire, as the initial material, is heated at a rate between 85 K/s and 100 K/s to a temperature above Ac3, specifically to a temperature of 860° C. and then held for 10 to 30 seconds, in order, on the one hand, to achieve complete conversion of the ferrite structure into austenite, and on the other hand, to produce a uniform distribution of the carbon in the austenite. To achieve a higher level of fine graininess, then, forming takes place at 860° C., the wire being ovalized in a first pass, rolled round in a second pass and then driven through a calibration nozzle. Afterwards, the wire is quenched and tempered again. The austenite is quenched to martensite in this known process with a microstructure which is not recrystallized. The strength or toughness properties of the wire produced using this known process can be improved with respect to increased vibration strength which is necessary for torsionally-strained spring elements.
German Patent No. DE 195 46 204 C1 discloses a process for producing spring elements from a heat treated steel in which thermomechanical treatment of the initial material is performed with the following steps:    a) The initial material of the steel is solutionized in the austenite range at temperatures from 1050 to 1200° C.,    b) directly thereafter the first forming is performed above the recrystallization temperature,    c) directly thereafter a second forming is performed below the recrystallization temperature, but above Ar3.    d) further forming and working processes are carried out below the recrystallization temperature, but above the Ar3 temperature, the holding time lasting one minute,    e) then, the rolled product is quenched to below the martensite temperature and finally tempered.
The austenite is quenched in this known process to martensite, likewise, with a microstructure which is not recrystallized by the forming according to the aforementioned steps c) and d). Since, in this known process, forming is performed not only below the recrystallization temperature, but holding times are even longer before quenching at high temperatures, there is otherwise considerable grain growth.
According to research by the applicant, the use of the known process leads to fine acicular, but highly textured martensite microstructures with strength and toughness properties which are good in the rolling direction. The known process is therefore suited for spring elements which are exposed to tensile/compressive loading in the rolling direction, as is especially the case in leaf springs.
In torsion-strained spring elements, such as helical springs and stabilizers, the direction of maximum loading does not correspond to the preferred direction of maximum strength which is produced by the aforementioned process. Rather, thermomechanical treatment which leads to an unrecrystallized, less recovered austenite grain is not suited for steels for torsionally loaded spring elements, and it does not lead to an improvement of vibration strength.